Since mobile devices such as mobile phones and notebook computers have become widespread, the role of secondary batteries is currently focused as power sources for the devices. These secondary batteries are required to be small and light and to have high capacities, performances that are less prone to deterioration even after repeating charge and discharge, and a high level of safety, and lithium-ion secondary batteries are most often used currently.
Negative electrodes of the lithium-ion secondary batteries are mainly made of carbon (C) such as graphite and hard carbon. Carbon can successfully repeat charge/discharge cycles, but large increase in capacities in future is not expectable because the capacities are already used until near the theoretical capacity. On the other hand, there are strong demands for increase in capacities of the lithium-ion secondary batteries, and materials for negative electrodes having higher capacities than that of carbon have been studied.
Examples of the materials for negative electrodes that can enable higher capacities include silicon (Si). Negative electrodes with Si have higher capacities because the absorption/desorption amount of lithium ions per unit volume is large. However, they have a disadvantage that large expansion and contraction of the electrode active materials themselves in absorbing and desorbing lithium ions promotes pulverization, which results in short charge/discharge cycle life.
Patent Literature 1 suggests the use of a high-strength binder for improving the charge/discharge cycle life. In Patent Literature 1, it is confirmed that the use of a polyimide as a binder can suppress expansion and contraction of the volume of Si particles during charging and discharging and can improve cycle characteristics. Patent Literature 2 discloses that cycle characteristics of a secondary battery are improved by incorporating a polyimide and a polyvinylidene fluoride in a binding agent of a negative electrode that contains silicon particles and/or silicon alloy particles as active material(s).